Enriching alumina content of cryolite fusions



Oct. 19, 1948. A. F. JOHNSON 2,451,494

ENRICHING ALUMINA CONTENT CRYOLITE FUSIONS Filed Jan. 28, 1947 2Sheets-Sheet 2 7gg-1f" ,M

@5y-Z. Z f 5139 Patented ct. 19, 1948 ENRICHING ALUMINA CNTENT OFCRYOLITE FUSONS Arthur F. Johnson, Longview, Wash., assignor to ReynoldsMetals Company, Richmond, Va., a

corporation of Delaware Application January 28, 1947, Serial No. 724,856

4 Claims. 1

This invention relates to enriching the alumina content of cryolitefusions in the electroiytic production of aluminum. The invention isespecially concerned with the electrolytic reduction of fusions ofalumina dissolved in cryolite and provides an improved apparatus andmethod for refining impure fusions of this character. My inventionprovides a meth-od of, and apparatus for, passing alumina-depleted:fusion from an electrolytic reduction operation to an aluminaenrichingand purifying operation including the addition of impure bauxitecontaining oxidic impurities to the fusion and the reduction of ironoxide therefrom at a temperature above the melting point of iron, andthe returning of aluminaenriched and purified fusion to the reductionoperation while passing through one or more stages of filtration.

One of the important features of my invention is the provision of abypass means for directing to the enriching and iron oxide reductionstage only a part of the alumina-depleted fusion, whereby only such partof the fusion is heated to the melting point 0f iron. The remainder ofthe alurnina-depleted fusion is thereafter mixed with the enriched hightemperature fusion to dilute the alumina and t-o decrease thetemperature, giving the fusion properties suitable for electrolyticreduction. I prefer to introduce crude calcined bauxite containing suchimpurities as iron Oxide, silica and titania in at least partiallyreduced condition together with a reducing agent including aluminum intothe fusion, and to agitate the fusion by any suitable means to intermixthe reducing agent therewith. I increase the temperature of the fusionbeing enriched, preferably by electric heating, to a temperature abovethe melting point of iron. At the temperatures employed I am abletoeffect an efficient reduction of iron oxide to molten iron and todissolve up to as high as 12% or 13% of alumina in only about one-halfthe aluminadepleted fusion, thereby reducing the amount of heat and thesize of the equipment necessary to 'remove the impurities and enrich thefusion. By mixing the high temperature enriched fusion with the otherportion of the alumina-depleted fusion, I produce a mixed fusion withthe desired percentage of alumina for electrolysis, for example, about7%, and at a temperature suitable for passage through a carbon filter.

In one aspect of my invention, I calcine the crude bauxite an operationin which I effect a substantial reduction of iron oxide. For example, Ipass into the calciner a reducing gas,

such as water gas (CO-f-Hz), to reduce as much of the iron oxide as iscommercially feasible. I prefer to do this at temperatures below themelting point of iron s0 that the reduced iron is in the nature ofsponge iron. In view of the pyrophoric character of the reduced iron, Iprefer to cool the calcined product under a reducing gas if it should benecessary to expose it to oxidizing conditions as in grinding, handlingand the like. In order to facilitate solution or" the alumina andseparation and sedimentation of the reduced iron, I grind the calcinedbauxite to a fine state, say, 1GO mesh. I mix powdered or granularaluminum with the ground bauxite, some carbon if necessary, and chargethis mixture into the aluminadepleted fusion where the temperature isabove the melting point of iron. The aluminum reduces most of the ironoxide remaining in the bauxite, the silica and the titania. The reducediron, silicon and titanium settle out of the fusion and form an alloy ofconsiderable value. This aspect cf my invention gives the significantadvantage that the iron oxide is very substantially eliminated beforethe fusion reaches the filters and the reduction of iron oxide in thelters is greatly minimized.

One of the characteristic features of my invention is the provision of acontinuous circuit through an alumina reduction stage, an aluminaenriching and oxide reduction stage and one or -more filtering stages,in which the fusion is elevated with pump means to obtain a suflicienthead for gravity ow, and a bypass circuit for bypassing alumina-depletedfusion from the reduction stage around the oxide reduction stage.

It is advantageous to elfect reduction in a sort of smelter providedwith means, such as electrodes, for heating to a temperature above themelting point 0f iron, means for agitating and mixing the fusion, suchas a refractory plunger, and a means for settling and separating thereduced metals. At the temperatures I employ in the reduction stage, Ican dissolve up to as high as 13% of alumina in the fusion and stillhave such fluidity that the fusion can be passed through a carbonfilter. I may add aluminum to the fusion being agitated and reduced,preferably powdered aluminum or ferro-aluminum, if necessary.Hereinafter, when I refer to the use of aluminum for reducing impuritiesI include metallic aluminum, such as powdered scrap aluminum or suitablealloys of aluminum such as ferro-aluminum. In this stage I also preferto effect a reduction of the silica and titania and a separation of theresulting silicon and titanium.

To this end I prefer to utilize the iron, either the reduced iron oradded iron, such as powdered iron or the iron of the ferro-aluminum, asa collecting medium for the silicon and titanium. The silicon andtitanium alloy with the iron and the resulting alloy is largelyseparated by settling out of the fusion. When ferro-aluminum is used,the residual ferro-aluminum dissolves silicon and titanium to form analloy.

Ihe enriched fusion containing up to as high as 13% of alumina and at ahigh temperature is incorporated in the main stream of aluminadepletedfusion blending the composition and temperature. The enriched fusion maybe filtered through a porous carbon lter before or after blending.

My invention provides an improved combination of apparatusadvantageously useful in carrying out the method of my inventioncomprising one or more electrolytic reduction cells, particularly awedge type Cell as described in my copending application Serial No.673,121, filed May 29, 1946, a smelter for the reduction and separationof iron and other metallic impurities, means for the bypassing of fusionfrom the reduction cells around the smelter, filters for the removal ofimpurities, and pump means, one for drawing fusion from the electrolyticreduction cell and owing it through the smelter and another forelevating the fusion to effect its flow through the final stages ofiiltering and back into the electrolytic reduction cells.

These and other novel features of the invention will be betterunderstood after considering the following discussion taken inconjunction with the accompanying drawings, in which: Y

Fig. 1 is a plan View of apparatus embodying my invention;

. Figs. 2 and 3 are sectional Views along lines 2--2 and 3 3,respectively, of Fig. 1, and

Fig. 4 is a vertical sectional flow View at the planes indicated in Fig.1.

The apparatus illustrated in the drawings comprises two main parts incombination-an electrolytic cell I one end portion of which is shown,and a feeding and purifying apparatus 2. Alumina-depleted fusion fromthe cell ows into the ypurifying apparatus through the duct 3 and thealumina-enriched fusion in the purifying apparatus returns to the cellthrough the duct 4. For matters of economy in the saving of heat, Iprefer to install both the electrolytic cell and the purifying apparatusin the earth with the upper surface approximately at floor level. I mayuse concrete lining in contact with the earth and insert inside of theconcrete lining 5 a steel shell B, the purpose of which is to preventmoisture from entering the inside of the apparatus.

The space between the concrete lining and the shell 6 is filled wtihrammed alumina 'I to serve as a thermal insulation. Inside the steelshell 6, a layer of porous carbon 8 is applied as thermal insulation.The entire inside of the apparatus within the porous carbon liningincluding all of the ducts and chambers is formed of car- -bon potlining 9 which is the material usually used for contacting cryolitefusions of alumina.

A removable cover l0 rests upon the upper flat surface of the apparatuspreviously described. The cover comprises a steel shell II with anexterior layer of porous carbon I2 and an interior layer of pot liningI3.

- :The ducts 3 and 4 are formed within bodies of pot lining I4 and I5extending from the relining apparatus into the electrolytic cell. In

order to prevent a short circuit of current through from theelectrolytic cell into the purifying apparatus, the ducts 3 and 4 passthrough electrical insulating blocks I6 and Il formed of a material suchas dense fused aluminum nitride brick prepared as described in mycop-ending application Serial Number 634,145.

Duct 3 connects with a heater H comprising a cylindrical upright chamber2G and a depending electrode 2|. of graphite mounted in the holder 22which is attached to a threaded member 23 by means of which theelectrode may be lifted or depressed to regulate the electrical energyin put. This electrode is used only when the fusion from the reductioncell is not sufficiently hot, say, below 950 C. The electrode ispreferably connected to alternating current.

Chamber 20 connects by duct 2li with a pump P which comprises an uprightcylindrical space 25 in which is mounted a loose tting cylindricalcarbon plunger 26 mounted on the shaft 2l'. This shaft is mounted withina sleeve 23 attached to the cover I3 and in the upper end a stufling box29 is mounted through which the metal shaft 33 embedded in the carbonshaft 2 connects to a piston, not shown, operating in the cylinder 3l.The pump, accordingly, comprises a refractory piston, a refractory stemoperating in a refractory cylinder, a stufling'box providing a gas-tightconnection for the stem, and the steel shaft connected thereto whichoperates in the cylinder 3l. In order. to regulate the flow of fusioninto and out of the cylinder 25 a refractory plug 32 is adjustablypositioned within the duct 24. The fusion is, accordingly, suckedthrough duct 3 and, by reason of plug 32, forced to ow out through twoducts, one duct 33 which leads to the smelter S and the other 34 whichleads to thelter F. The smelter S comprises an upright cylindricalchamber 35y and a depending electrode 36 preferably formed of graphite.This electrode isy suspended in the holder 3l slidably mounted in theYguide rods `Si! and 39 and which is adjusted in its upward or downwardposition by the screw lli to regulate energy input. The downwardlyslop-ing feed duct 4I extending from a hopper or other suitable storagespace, not shown, enters the chamber 35 for the introduction of calcinedcrude bauxite and reducing agent into the chamber. The duct il comprisesan electrically non-conducting sleeve of fused alumina 42 and a steelsleeve d3 in which is mounted a worm or other mechanical feeding device,not shown. Both the feed duct 4I andV the duct 33 enter the cylinder 35near the top. The bottom portion of the cylinder 35 is contiguous with alateral chamber la which has an inclined bottom i5 sloping Yinto thechamber 35. The chamber 44 is also contiguous with an uprightcylindrical chamber i6 which has a duct lll leading into the lter ll'.The cylinder 46 is part of a mixer M comprising a carbon plunger 53mounted on a carbon shaft 5I, the upper end of which is attachedto asteel piston rod 52 which passes through the stufling box 53 andv isconnected to a piston, not shown, in the-cylinvessel. The filter f58 ispositionedfto receive fusion overflowing from duct 41. The graphiteheating electrode '6l is supported in a holder similar to that forelectrode 2| and passes through the gas-tight s'tufiing box 62 .in thecover lil. The bypass duct 34 connects the cylindrical space 26 of pumpP with the cylindrical space 51 of the filter. The fusion pumped out ofspace 26 flows in part through duct 33 into the smelter S and in partthrough duct 34 into the filter F. It is sometimes advantageous to usethe electrode 6|v to increase the temperaturel of the fusion tofacilitate filtering. Preferably the mixed fusion is ltered at atemperature of about 11200C. I

The mixed fusion enters the duct 65 and ows into the heater H'. Thisheater comprises an upright cylindrical chamber 66 in which thedepending graphite electrode 61 is mounted. This electrode passesthrough a gas-tight gland 68 and is supported in a holder similar to theholder for electrode 2| and may be adjusted upward and downward by thatapparatus.

That portion of the cover which surrounds pump P and filter F issomewhat elevated to provide for the relatively long length of strokeused in the piston of that pump and also to facilitate removal of thepump.

The fusion from heater H flows through duct 16 into the mid-portion ofcylindrical space 1l of pump P. The carbon piston 12 attached to thecarbon rod 13 is reciprocable in its cylinder. The rod 13 passes outthrough the stuffing box 14 which is gas tight and the steel piston rod15 is threaded into the rod 13. The piston rod 15 enters the cylinder 16in which a piston (not shown) on the rod 15, with a relatively longstroke, operates piston 12. 'I'he carbon plunger or valve 11 isadjustably mounted in the duct 10 to regulate the ilow therethrough. Thebottom of cylinder 1| is connected to the upright duct 18 extendingthrough to the top of the cover, the upper end of which is closed by thelid 19. The lateral duct 80 connects to the upright cylindrical space 8|of the lter F'. This lter comprises a hollow cylindrical porous carbonfilter 82 which is positioned to receive fusion overflowing from theduct 80. The fusion that passes through the filter enters the duct 83 atthe bottom and flows upwardly until it meets the lateral duct 84 whichenters the upright cylindrical space 85 of the filter F". The duct 83extends to the top of the cover for cleanout purposes and is protectedfrom the atmosphere by the removable lid 86. Fusion in the lter F" isheated by the graphite electrode 81 which passes through the gas-tightstuffing box 88 and is adjustably supported by a holder similar to thatfor electrode '21.

In the upright cylindrical space 85 of the lter F" is mounted the hollowporous carbon cylinder 96. Duct 84 is arranged to discharge fusion intothe hollow center of cylinder 96 and the fusion which passestherethrough enters the upwardly inclined duct 4 and returns to theelectrolytic cell l. The fusion in iilter F". may also be heated whenrequired with an electrode 91 which passes through the gas-tight gland98 to a suitable adjustable holder such as that for electrode 2l. Therefining apparatus and electrolytic cell are so arranged that the normallevel of fusion in the cell where it returns through duct 4 is about Iinches below the top of the cell, whereas the fusion in the cell whereit enters duct 3 is about I9 inches below the top of the cell. This tionin any objectionable amount.

The various heating electrodes comprise a relatively large diameterportion which is held by the holders and a relatively small diameterportion which enters the fusion. This provides an velectrode with lowlead-in heat loss and a small diameter lpart of higher resistance forheating the fusion. The stufling boxes are preferably all alike and karesuici'ently tight to prevent objectionable gas leakage. They may bestuffed with asbestos or metal packing. The pistons 26, 50 and 12 `andtheir connected stems 21, 5l and 13 are made of carbon having highstrength and relatively ,low heat conductivity so that the steel shaftsconnected thereto will not become overheated.

The carbon filters may be of any suitable construction such as packedgranular carbon or porous carbon. As illustrated, the lters are formedcylindrical vessels of porous carbon. Since most of the iron, siliconand titanium are removed before the fusion leaves the smelter, theiilters yare not unduly burdened with the reduction of iron oxide orwith the trapping of particles of reduced metals. The filters serve theimportant function of chemical scrubbers for the reduction of iron oxideto low percentages and the holding of the reduced iron in the pores.

The Various heating electrodes are provided more or less for emergencyuses, such as when portions of the circuit in the purifying apparatusneed supplemental heat.

The alloy which accumulates on the bottom 44 may be removed by liftingoff the electrode unit 36 and freezing the alloy on a lifting member, asdescribed in my copending application Serial Number 685,695, led July23, 1946.

The fusions undergoing treatment contain alumina dissolved in cryoliteas is usual in the Hall process. Frequently other salts are added to thefusion to lower the melting point and to increase the solubility ofalumina, such as other fluorides, especially luorspar.

I claim:

1. In the electrolytic reduction of alumina from cryolite-containingfusions of alumina, the improvement which comprises withdrawingalumina-depleted fusion from the electrolytic reduction operation,adding crude bauxite containing oxidic impurities including iron oxideto a portion of the withdrawn alumina-depleted fusion in amountsufficient to bring the alumina content of that portion of the withdrawnfusion up to as high as 13%, heating the enriched fusion in contact witha reducing agent from the class consisting of aluminum, aluminum alloysand carbon to reduce the iron oxide to iron while at a temperature abovethe melting point of the reduced iron, separating molten iron from thefusion, mixing the alumina-enriched fusion from which the molten ironwas separated with another portion of the withdrawn alumina-depletedfusion, said another portion of the withdrawn alumina-depleted fusionbeing in amount suiicient to produce a resultant fusion having analumina content of about 7%, passing the latter fusion through a carbonlter and back into the electrolytic reduction operation.

2. The improvement in the electrolytic reduction of aluminum fromcryolite-eontaining fusions of alumina set forth in claim 1 in which theresultant fusion is passed through the carbon filter while at atemperature of about 1200 C.-

3. The improvement in the electrolytic reduction of aluminum fromcryolite-containin-g fusions of alumina set forth in claim 1 in whichthe re- 10 ducing agent is aluminum.

4. The improvement in the electrolytic reduction of aluminum fromcryolte-containng fusions of alumina set forth in claim 1 in which theamount of crude bauxite which is addedrto the first-mentioned portion ofthe Withdrawn alumina-depleted fusion is in amount sufcient to increasethe alumina content thereof to about ARTHUR F. JOHNSON.

8` REFERENCESY'CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS

